There are a variety of fuel sources from which energy can be extracted for useful work such as generation of steam, heat, and generation of electricity. Fuel sources generally are cost components that incur expenses for procurement, processing for use as a fuel, transport, storage, and use. Such fuels include coal, natural gas, and the like.
Alternative fuel sources include energy-containing combustible products such as members of the plant kingdom which have been processed, for example, fibers from agricultural processing. Fibrous byproducts often result from agricultural product processing systems. The term “byproduct” is used to refer to a secondary or incidential product arising from a manufacturing process of agricultural products, for example processed agriculture residues (PAR) including distiller grains and rapeseed. Such byproduct may have some economic value or in a worst case, no economic value and therefore is a waste. For example, ethanol production using distiller grains generate as a byproduct fibrous materials that are substantially depleted of carbohydrates. Rapeseed processing generates an oil suitable for biodiesel suitable for powering motor vehicles but also produces crushed rapeseed as a byproduct. Use of fiber byproduct for extraction of energy provides an economic benefit from a material that otherwise may have disposal costs and no or limited other practical benefit. Fiber is used herein to refer to any material derived from a member of the plant kingdom that has been physically separated or at least partially depleted (i.e., to less than about 40% d.s.b. total) of sugars, starch, protein, and germ. Fiber can be burned or combusted to provide energy; depending on the plant source, the fiber's composition, and its water content, it typically has a higher heating value (HHV) of about 7000-10,000 BTU/lb (dry basis). Other plant materials, such as corn germ, can have HHVs as high as 15,000 BTU/lb (dry basis). Therefore, theoretically, fiber or other plant material could be burned in order to wholly or partially power industrial processes.
Fibers however have generally not been used as an energy source. Most fiber contains relatively high levels of ash (inorganic ions, such as elemental or compounds of phosphorous, calcium, magnesium, sodium, and potassium). For example, typical corn kernel fiber contains about 4 percent dried solid basis (DSB) ash, of which phosphorous is the most common element (total ash containing about 40 WT % P2O5). Distiller grains contain about 5 percent DSB ash, with total ash similarly containing about 40 WT % P2O5 Fiber ash generally has a relatively low fusion or melting point, meaning that at high temperatures the ash is molten, and will form slag on refractory or metallic surfaces of a furnace, boiler or flue gas stack if the molten ash contacts these surfaces. Ash at temperatures below its melting point is generally in the form of small, generally irregularly shaped, solid particles.
One byproduct of bioethanol processing using corn or other grains is distiller grains (DG). The DG byproduct generally comes from the process as wet cake at about a 70% moisture content. Heretofore DG products have been used as a blending material for cattle feed, and thus derive some economic value from the byproduct. Such use of this byproduct however has additional costs. The moisture content of the DG must be reduced from about 70% to about 10% after which the material is customarily referred to as dried distiller grains (DDG). It is impractical to dry this material naturally; accordingly, the drying of this byproduct to be a useable feed blended material incurs additional fuel and processing costs.
As an alternative to a cattle feed blend, the energy contained in the DDG could be extracted through combustion. Fluidized bed combustion chambers however are impractical for combusting this byproduct as a fuel. The temperature of the fluidized bed would have to be carefully monitored in order to make adjustments in temperature in the event that the DDG ash-agglomerates in the fluid bed and forms a slag coating on the bed material. Slag coatings create the potential for a “frozen bed” or at least the formation of large agglomerated masses (“clinkers”) within the bed. This slag coating detracts from the heat extraction, reduces the efficiency of the combustion, and after significant buildup, prevents the fluidized bed combustor from working properly. Clearing a frozen bed or removing large agglomerations of material is time consuming and difficult work that involves cessation of the combustion chamber operation.
In addition, the relatively high moisture content of the DG prevents the DG from being stored for subsequent usage. The moisture causes the DG to ferment in storage, and yet potentially may cause a fire in the storage chamber.
DDG (having a moisture content of 12% weight or less) however is readily storable. Periodic cessation of combustion processes, such as for example, during cleaning of a combustion chamber that combusts DDG, would not create a storage problem, because additional DDG received from the ethanol distilling processing can be placed in storage silos for subsequent use.
There are drawbacks however to the use of DDG as a combustion fuel. Because this byproduct has a high fouling potential, it is believed that conventional furnace combustion chambers are unsatisfactory for achieving a low furnace exit gas temperature to preclude slag formation. The high fouling potential is due in part to constituents in the ash that have low melt temperatures relative to the operating temperature range of boilers. Also, high fuel-bound nitrogen can increase nitrogen oxide (NOx) emissions.
Similarly, cake from crushing rapeseed for oil production has similar moisture and ash content problems, and has similar elemental analysis for potential use as a fuel.
Accordingly, there is a need in the industry for extracting energy from processed agriculture residues (PAR). It is to such that the present invention is directed.